Combined heat and power (CHP) systems have been utilized in many forms for over 100 years. The most common are fossil fuel fired systems that use, for example, steam turbines, gas fired turbines and internal combustion engines to produce power. The reject heat from these systems can be used for a wide range of applications such as heating, cooling and in some cases where the waste heat temperature is high enough can be used to drive a second cycle. Most of the focus for traditional CHP systems has been large fossil fueled fired systems connected to district heating grids. Over the past 30 years the focus moved to smaller distributed CHP systems where the heat or power generated could be better utilized by the end user. These systems have also been fossil fueled fired systems that commonly use small gas turbines or reciprocating engines to produce power along with usable waste heat from the cycle. Other systems that use Organic Rankine Cycle have also been used although the quality of the waste heat is relatively low which limits the applications for the heat.
More recently, the focus to use renewable organic waste streams for fuel has been predominant and progressing the technology. Large biomass and municipal solid waste to power systems have been in operation for many decades. The ability to utilize a wide variety of organic sources in a small CHP system (for example, less than 1 MW electric) has been challenging for a number of reasons. There have been many small organic to power conversion technologies that have been utilized to varying degrees of success. Gasification of organics into syngas has been one approach to convert a solid fuel into a hydrocarbon gas for combustion in traditional power systems. Unfortunately, these systems can be costly, especially when scaled to smaller scale applications. In addition, the organic feed stock can present particular challenges in application. For example, depending on the gasification method and conversion efficiencies, the potential energy available in some organics suffers from losses that have an economic impact on the cost of power and heat. Gasification of mixed organic residues is particularly problematic for many gasification systems.
Another method applied for small CHP applications has been to use direct combustion of the organics through an appropriate combustor and using the heat through a heat exchanger to drive an externally fired engine. Traditional externally fired systems include Stirling cycle, Steam Rankine, Organic Rankine, and super critical CO2 cycles. In all of these systems the temperature of the reject heat affects the cycle efficiency. The higher the reject heat temperature the lower the power efficiency. With the exception of the steam cycle the other thermodynamic cycles typically lose efficiency when producing even hot water at 90° C. However, with the steam cycle, the deficiency is the complexity and cost associated with a high pressure steam circuit in a small application.
Another method that has been employed more recently is to use an open Brayton cycle gas turbine and introduce heat indirectly through a heat exchanger. Several systems have been tested where a small turbine has been coupled to an organic combustion system. In these systems ambient air is compressed in the compressor of the turbine and then directed to a recuperator to preheat the compressor air. The compressed and preheated air is then directed to the hot heat exchanger to be heated by the organic combustion system. The highly heated air is then expanded in the turbine to produce work to turn a generator and generate electricity. Continuing the cycle, the hot turbine gases are used for preheating in the recuperator as mentioned earlier. The turbine exhaust hot gases may then be exhausted or directed to a further heat exchanger where combustion air is heated with exhausted combustion gases and directed to the combustion process. In all of the configurations of the prior art the turbine utilizes a recuperator to preheat compressed ambient air. Furthermore, most existing systems employ a combustion air pre-heater to recover heat from the combustion gases as well as the heat remaining in the turbine exhaust. While these approaches improve the thermal to electric conversion efficiency, it requires multiple heat exchangers and complex piping for routing of the gases in the circuit which may reduce the overall system efficiency.
The effect of the recuperator is to raise the temperature of the compressor air prior to the hot heat exchanger. Heat is extracted from the expansion turbine to preheat the compressor air. The compressor air is further heated by the hot heat exchanger where heat is extracted from the combustion gases. Utilizing the recuperator reduces the size of the hot heat exchanger. However, it also reduces the amount of heat energy extracted from the combustion gases. In order to achieve high thermal electric efficiency, it is desired to recover as much heat as practicable from the exhaust combustion gases. Since the combustion exhaust gases cannot be directly fed back to the combustion system, a combustion air pre heater is necessary to recover the heat from the exhaust gases. These losses can be reduced by using the turbine exhaust air as the combustion air but this still requires the air preheater.
Another challenge for existing systems is related to controlling the temperature of the combustion gases entering the hot heat exchanger. For applications using a Brayton cycle gas turbine it is desirable to be able to deliver the compressor (or expander) air at the design inlet temperature of the turbine. In many cases this temperature can be as high as 950° C. To achieve this inlet air temperature, heat exchangers need to operate near the maximum design temperature limits. In addition, it is important to be able to maintain a constant temperature at the heat exchanger. Overheating and temperature variations could cause stresses on the heat exchanger as well as the expansion turbine while under heating would cause lower inlet temperatures reducing power and efficiency. Variations in temperature can be a significant concern when combusting mixed fuels that can have very large differences in heat values.
Existing systems also commonly utilize a constant volume feeding system to deliver the solid fuel into the combustion chamber. There is no system to determine the heat value of mixed solid fuels as it is fed into the combustion chamber. As the fuel is burned the higher heat value fuel will cause an increase in temperature of the exhaust gases while the lower heat value fuels will have the opposite effect. As a result, mixed solid fuels create varying gas temperature flows which cannot readily be corrected by adjustment of the feed system.